"I'm surprised at how heavy it is. 5 kilos, and that's without lens or battery or all the other things it needs."

I would say it's mostly a cultural thing, as people in the movie business are used to heavy hardware. Five kilos isn't heavy at all for them. If you search for "Arriflex", the camera that defined the standards in the movie industry, you will undoubtedly find images of some camera configurations weighing in at 90Kg+.

I'm not saying that some weight loss wouldn't be welcome in many situations, but to succeed in that market, it is crucial to be compatible with the standard heads for tripods, dollies and cranes, which are all designed and balanced for the older material.